1. Field of the Invention.
This invention relates to the field of semiconductor devices and more specifically to an etchback process designed to etch a blanket layer of a refractory metal such as tungsten (W).
2. Prior Art.
In the manufacture of semiconductor devices, it is normally necessary to make contact to device regions underlying a dielectric on the surface of the silicon substrate. This is accomplished by first forming an opening or via (contact via) in the dielectric over the region to be contacted, and next filling the contact via with a conductive material.
In addition to filling the vias with a conductive material, it is necessary to electrically connect certain device regions with others, as well as to provide for electrical connection to external leads. These requirements are met by forming a wiring layer on the surface of the substrate. The wiring layer is formed by depositing a conductive material on top of the dielectric layer in which the vias have been formed. The conductive layer is then masked and etched to leave continuous lines of the conductive material necessary to make the appropriate connections to the device regions of the substrate. These lines are known as interconnects.
Several conductive materials can be used as a contact via fill. In larger geometry devices, the via fill and interconnect formation are accomplished simultaneously. Aluminum (Al) is deposited on the entire substrate, as well as in the vias. The areas over the vias and interconnects are then masked with photoresist and the aluminum is then etched from the remaining areas, leaving the vias filled with aluminum as well as forming interconnects on the surface of the dielectric layer.
As geometries have shrunk to submicron levels and devices have become more densely packed on the substrate surface, the openings or vias to the device regions to be contacted have increasingly greater aspect ratios (ratio of height to width). Aluminum deposition alone has proven to be inadequate in devices with high aspect ratios. The problems encountered include poor step coverage, poor contact integrity, and inadequate planarity.
To overcome these shortcomings, tungsten and other refractory metals are being used as a contact filling for devices with submicron contacts before aluminum deposition and patterning. For example, a blanket tungsten layer (tungsten "film") is deposited followed by a blanket etchback to remove deposited tungsten from the surface of the substrate, leaving a tungsten filling or plug in the contact openings or via. The aluminum layer is then deposited, covering the substrate surface including the filled contact vias. This aluminum film is then patterned and etched to form the interconnects.
One problem encountered with the tungsten process is the "micro-loading effect", where the tungsten etch rate drastically accelerates in the contact opening when the bulk of the film has been removed from the surface of the silicon substrate (that is, when the film "clears"). The result is that the contact fillings or plugs are recessed below the surface of the dielectric and are sometimes completely removed by the end of the etch. Because of the micro-loading effect, it is extremely difficult to obtain uniform contact fillings while ensuring that the bulk of the metal is completely removed from all other areas of the substrate surface. Slight non-uniformities in the metal thickness or etching process over the surface of the wafers will cause the bulk metal on the surface of the substrate to be etched in some areas of the substrate before others. If even a slight overetch is employed to ensure complete etching of the bulk metal from all areas of the surface, the metal filling the contact openings will begin to etch rapidly in those regions of the surface where the bulk metal clears first. This results in extreme variations or non-uniformities in the filling levels of the contact openings. The filling in the contact openings located in the area where the bulk metal cleared last will be completely unetched-that is, the contact openings in this area will be completely filled with tungsten, while the filling in the contact openings in areas where the bulk metal cleared earlier will be etched to different extents-some will be recessed slightly below the surface, others will be recessed to greater depths and some will be missing entirely.
A three step blanket etch process utilizing a timed etch in a gas mixture comprising sulfur hexafluoride (SF.sub.6), oxygen (O.sub.2) and helium (He), a second etch in a gas mixture comprising SF.sub.6, chlorine (Cl.sub.2) and He, and a short timed overetch in a gas mixture comprising Cl.sub.2 and He is described in U.S. Pat. No. 4,980,018 entitled "Plasma Etching Process for Refractory Metal Vias", which patent is assigned to the assignee of the present invention. The above-referenced patent describes a process for overcoming the microloading effect and provides for complete etching of a blanket tungsten film while leaving the contacts uniformly filled with tungsten.
An additional problem in prior art processes is residue build-up on the reactor walls and electrode. The chemical make-up of the residue depends on the etchant gas used as well as the material being etched. Many prior art processes use etchant gases containing carbon such as CF.sub.4, CBrF.sub.3, CF.sub.3 Cl and CF.sub.2 Cl.sub.2 for example. These etchants are a source of carbon contamination. Many prior art processes use sulfur hexafluoride (SF.sub.6) which causes the residue to contain sulfur. This in turn leads to the formation of H.sub.2 S upon opening the reactor from the reaction of the sulfur in the residue with water in the air. The H.sub.2 S has a pungent odor which is unpleasant for workers in the immediate area. It is also found that other non-volatile compounds, depending upon the metal etched, comprise part of the residue. For example, when etching titanium with the prior art processes, various oxides and halides of titanium are formed on the electrode.
In addition to the bad smell associated with the sulfur residue, the presence of any residue on the electrode degrades the systems performance. The residues clog the gas distribution holes which affects gas flow. Therefore, the residue must be scraped off of the electrode frequently, thereby increasing periodic maintenance requirements. In addition, this physical scraping of the residue reduces electrode life, increasing the upkeep cost for the system.
It has been noted that performing an etch in an RIE process using nitrogen trifluoride (NF.sub.3) leaves no nonvolatile residue. In "CVD Tungsten Contact Plugs by In Situ Deposition and Etchback", Gregory C. Smith, V-MIC conference, Jun. 25-26, 1988, a blanket CVD deposition of tungsten followed by an etch in NF.sub.3 in an RIE system is described. Similarly, in "Ion-bombardment-enhanced plasma etching of tungsten with NF.sub.3 /O.sub.2 " W. N. Green et al., J. Vac. Sci. Technol. B6 (5), Sep/Oct 1988, a process using etchant gases of NF.sub.3 and O.sub.2 in RIE conditions in order to avoid surface residue is utilized to etch tungsten. However, RIE etching is in general not as manufactureable an etching process as other methods, primarily because of gate charging caused by the high DC bias of RIE.
Another problem encountered in etching refractory metals off of a dielectric is that the dielectric layer itself can be etched or otherwise degraded. One problem encountered is texturing or roughness of the dielectric surface caused by the tendency of the etch to replicate the tungsten film grain structure into the underlying dielectric surface. Other phenomena which occur to the dielectric surface include the formation of "spires" which are tall, thin (approximately 0.5 micron high and 0.1 micron in diameter) columns of tungsten remaining on the dielectric surface after the etch. These spires are probably caused by micromasking. A similar effect, called "grass" formation, is when the same type columns of the dielectric itself are formed, with no tungsten on top of the column of dielectric.
What is needed is a refractory metal etchback process that does not suffer from the micro-loading effect, does not add additional steps or complexities to the process, and has a sufficiently low throughput time to allow for high volume IC production. It is further desirable that the etch process minimizes residue formation on the reactor electrode, thereby reducing reactor maintenance requirements.